Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200601

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
201 AM EDT Sun Aug 20 2017

High pressure will build across the forecast area and bring
relatively dry conditions through Sunday. Then a warming and
moistening airmass will build in, bringing hot and humid afternoons
Monday through Wednesday. A cold front is expected to pass through
Wednesday night and usher in cooler and drier air that will last
into next weekend.


As of 155 am: Very quiet weather persists through the near-term with
a post-frontal dry airmass in place. The front that passed through
yesterday will remain stalled to our south, but with the ridge axis
building in overhead the front will wash out somewhat, and moisture
will lift back north as surface winds swing around to the southeast
during the day on Sunday. This morning`s low temps should be very
close to climo.

later today, the heat will slowly increase as will the humidity. Low
level winds out of the S/SE will result in some orographic upglide,
and at 700mb, though fairly weak, as a very minor shortwave passes
over the area. This should result in a fairly typical diurnal
pattern to convection over the mountains, and should be enough for
isolated thunderstorms to develop. Storms probably won`t move much
but without enough time for recovery in deep-layer moisture, heavy
rain concerns are minimized. Other concern will be the increasing
heat with thicknesses increasing aloft as well as subsidence from
the building ridge, with highs creeping toward the mid 90s south of
the I-85 corridor. Luckily again without much time for low level
moisture recovery, heat indices will only be a degree or two above
the actual temps, but for residents and visitors across the area,
still a good idea to be aware of heat precautions.


As of 2:30 PM EDT Saturday: The western extension of the Bermuda High
Pressure System will hold sway through Monday. H5 heights actually
are on the increase into early in the new work week. Meanwhile, the
old frontal boundary washes out and is essentially replaced by an
establishing SE flow from the surface up to H5 MB.

In the very short term, isolated ridge top convection Sunday evening
will end with the loss of heating and instability. We will carry a
slight chance for an hour or two after sundown Sunday evening.

The broad SE flow has made a couple of notable shifts in the
forecast for Monday. With dewpoints on their way back up, we have
noticed an increase in moisture both from the surface to H85 and
also the surface to H5. The first area to discuss will be the NC
mountains and perhaps far NE Georgia. The suite of models are
allowing clouds to develop, along with a fair amount of instability,
which should lead to either scattered or widely scattered showers
and thunderstorms midday Monday forward. Terrain impacts will also
be in play.

The second area is the moisture advancing north from the midlands on
Monday. We will edge up the cloud forecast in our upstate south
piedmont counties and also have a small chance of afternoon showers
or  thunderstorms. The remainder of the area will have some fair
weather cumulus, with perhaps some higher level clouds on top.

I still suspect there will be some "give and take" with respect to a
sharp cloud/precipitation line nearby on Monday. Bottom line, minor
details could change the direction of the forecast.

Any convection Monday should wane, although we re-introduced another
chance of showers in our south mountains of NC toward daybreak
Tuesday. This is in response to the flow turning southwest, which
will initiate upglide on that side of the terrain.

Moving forward, the overall pattern will be in flux. On Tuesday there
should be a greater coverage of showers and thunderstorms, still in
the chance category, but increasing in areal coverage. A prefrontal
trough, ahead of the primary approaching cold front, may lend itself
for a thunderstorm focus Tuesday as well.

Temperatures will continue to remain elevated, while dewpoints start
another surge higher. As a result heat indices will be flirting in
the middle and upper 90s in parts of NE Georgia (outside the
mountains), upstate South Carolina and the western piedmont of North
Carolina, both Monday afternoon and Tuesday afternoon.


As of noon Saturday: Consensus of GFS and EC continues to indicate
an amplified upper pattern developing in the middle of the week,
reaching its greatest magnitude around Thursday. As the component
trough digs over the East, a fairly strong cold front will push
into our region late Wednesday. At the present time it looks fairly
likely this front will focus deep convective development, given that
it will be moving into a hot and humid airmass associated with a
subtropical ridge. With 20-25 kt of 0-6 km shear we can`t rule out
persistent linear storm structures, though dry air aloft appears to
be lacking, which may mitigate the severe wx threat to some degree.

Given the amplified pattern, unsurprisingly a large sfc high
builds into the area following the front. While some guidance
depicts precip returning Thursday afternoon while the front is
still on our southern fringe, most indications are for subsidence
to have taken hold by that time, and therefore depict suppressed
convection. By Friday the high should have moved onto the Eastern
Seaboard, resulting in a cool wedge setup which persists into the
weekend, with deep convection looking even less likely. However,
easterly flow still warrants a low PoP in the high terrain. Max
temps will be 3-5 degrees below normal Thursday and into the weekend
on account of the wedge, with dewpoints in the upper 50s to lower
60s making for particularly pleasant conditions.


At KCLT and elsewhere: The main concern for this forecast cycle will
be the potential for fog and/or low stratus, particularly in the mtn
valleys and near lakes (i.e., KAVL and KHKY). There are mixed
signals in model statistical guidance and in observational data in
terms of whether restrictions will develop at these sites, but opted
to err on the side of climatology, and tempos for IFR (at KAVL) and
lMVFR (at KHKY) have been included between 10-12Z. Otherwise, VFR
conditions are expected to persist through the period. Chances for
high terrain convection will be slightly improved today vs.
yesterday, but coverage will be very sparse and no mention of
convection is warranted at any of the terminals this afternoon/
evening. Otherwise, light N to NE winds should turn south of due
east this afternoon.

Outlook: Chances for diurnal convection and morning mtn valley
fog/low stratus gradually increase during early-to-mid week, with
the best chances expected Wednesday, as a cold front pushes into the
area. Chances for restrictions and diurnal convection may diminish
again during late week.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  91%     High 100%     High 100%     High 100%
KGSP       High  91%     High 100%     High 100%     High 100%
KAVL       High  83%     High  93%     High 100%     High 100%
KHKY       High  83%     High  94%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  89%     High  94%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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